The invention relates to a flow duct structure for reducing secondary flow losses in a bladed flow duct for example, in a fluid flow engine such as a turbo engine.
British Pat. (GB-PS) No. 1,132,259 discloses a system of this type in which a flow channel has a flow cross-sectional area which becomes narrower in the flow direction from the entrance end toward a duct throat located in the exit side or in the rear half of the duct. The duct is provided with an arched zone in the form of a longitudinally extending depression including a down slope in the direction across the flow direction toward the suction side of a blade. This type of arched depression provides an increase in the flow cross-sectional area which increases the static pressure whereby the difference of the static pressures between the blade pressure side and the blade suction side of the duct is reduced. Stated differently, such a depression reduces the transverse pressure gradient in the direction across the flow direction, that is in the circumferential direction.
It has been found that secondary flow losses or so-called wall losses or eddies in the duct are caused by the fact, among other causes, that the boundary layer formation at the suction side of a blade is increased. Such increase in turn appears to be due to the fact that the boundary layer entering into the duct and the boundary layer being generated along the duct walls in the form of a frictional layer along the walls is driven by the transverse pressure gradient and reaches the blade suction side in the form of a cross flow or secondary flow which extends at a slant relative to the primary flow. The just mentioned resulting boundary eddies strongly affect the quality of the energy transformation, especially in connection with blades having a low aspect ratio which is the blade span or length divided by the chordal width. These boundary eddies or secondary flow losses are reduced by the above mentioned reduction in the transverse pressure gradient.
However, the above mentioned arched zone in the form of a depression is disadvantageous because its down slope is convex from the duct entrance to the maximum or rather to the deepest depression area located in the zone of the above mentioned rear throat. Thus, fluid flowing from the duct entrance to the duct throat is accelerated in addition to its normal acceleration. Accordingly, the pressure increase achieved by the arched depression is limited essentially to the throat zone in which the flow speed is decreased due to the increase of the flow cross-sectional area caused by the lowest zone of the arched depression and by the concave curve of the depression. On the other hand an increased pressure drop occurs along the zone of the mentioned amplified acceleration, whereby the respective transverse pressure gradient is increased. Accordingly, the mentioned decrease in the transverse pressure gradient is achieved substantially only in the zone or area of the duct throat which zone is relatively short and besides is located in the rear half of the duct. Accordingly, only a relatively small reduction of the secondary flow eddies or secondary losses is achieved. Another disadvantage of the arched depression is seen in that in most instances the depth of the depression cannot be made to such an extent as would be desirable due to the given structural facts of the respective engine, particularly since the blade platforms or blade carriers do not have the sufficient thickness required for this purpose.
British Pat. No. 944,166 discloses a turbo engine rotor having a circumferential surface which is partially arched outwardly between two axial flow blades. This arched portion extends primarily in the pressure side zone of the blades and in the downstream direction the depression merges gradually into an inwardly arched or depressed contour relative to a cylindrically extending circumferential surface of the duct bottom or duct floor.